1. Field of the Invention
The present invention relates to a process and apparatus for separating liquids into more concentrated and less concentrated portions for a variety of applications including treatment of industrial wastes, separation of plating solutions, and concentration and refining of chemicals.
2. Description of the Prior Art
It is often desirable in, for example, plating processes to concentrate the plating chemicals which accumulate in the plating rinses for purposes of returning the concentrated portion to the plating solutions and, alternately, returning the remaining less concentrated liquid portion to be reused as rinse. A number of advantages accrue from such a process. For example, recovery of the plating chemicals is not only economically desirable, it may reduce or eliminate the discarding of contaminents which are ecologically harmful. Reuse of the less concentrated portion reduces water consumption and accordingly the prior art has sought economical ways to reuse the solution from plating rinse baths.
Various methods and apparatus have been proposed in the past for separating and recovering such solutions and some of these have used "reverse osmosis units" to concentrate a portion of the chemicals from the rinsing baths. As used herein, the term "reverse osmosis unit" means a device in which a pressure is applied to a solution to be concentrated of sufficient magnitude to cause the solution to pass through a membrane to a less concentrated side while the chemicals being concentrated are restricted from passing through the membrane. It is intended that the term include the rather analogous process referred to as "nanofiltration" (sometimes referred to as separation of dissolved larger molecular weight chemicals) which, like reverse osmosis, operates to separate an input solution into two outputs one of which is more concentrated and the other of which is less concentrated than the input solution. While reverse osmosis units have a number of advantages, they also have the disadvantage that membrane performance and lifetime is reduced when processing liquids of high concentration at extremes of pH, oxidizer content or that otherwise react with the membrane constituents. In addition, the resulting solution of chemicals is normally at an insufficient concentration to be returned directly to the plating bath. To solve the second problem, it has been suggested that a concentrate from a reverse osmosis unit be reconcentrated in a second reverse osmosis unit and, if necessary, the concentrate from the second reverse osmosis unit be again reconcentrated in a third osmosis unit so as to bring the concentrate to a sufficient concentration for return to the plating bath. Examples of such prior art techniques may be found in the Ogawa et al U.S. Pat. No. 4,255,255 and the Gross et al U.S. Pat. No. 3,836,457. Such prior art systems, however, employing more than one reverse osmosis unit, become extremely expensive to operate and have generally been found to be economically unsound. Furthermore, the problem of short membrane life has heretofor remained unsolved.